I want to generate my custom invoice sequence as per current financial year.

For exa. -- when i generate invoice on "2013-11-08" date then invoice no. generate like this - SAJ/13-14/P/0001
and now if financial year get change then it'll be changed like this -- SAJ/14-15/P/0001.
Look at the highlighted part in Invoice number.

So how can i generate financial year like this ??? OR how can i achieve this type of invoice number format ??

To generate custom invoice number, you have to make changes in "ir_sequence.py" file. This file is located on following path - C:\Program Files\OpenERP 7.0-20130917-231058\Server\server\openerp\addons\base\ir

Note: Above path may vary in different environment.

Now Open that file and find this method - _interpolation_dict(self):
do changes in this method as per your requirement. (Though it is not advisable to make changes in this type of file because when you update openerp then this type of changes might be lost.)
This function return dictionary, and then whatever result is there, you can use it in invoice sequence.

e.g. In openerp sequences we can insert current financial year - SAJ/%(cur_fin_year)s/ instead of SAJ/%(year)s/...
where cur_fin_year is my custom defined key in above method.